U.S. Pat. No. 4,027,669 describes a thermoplastic syringe barrel with an integral tapered adapter surrounded by an internally threaded locking sleeve. This sleeve is longitudinally slotted to aid in strip ejection from a mold during its manufacture.
Integral locking sleeves that are strip ejected are more economical to manufacture than separately molded sleeves which must be subsequently attached to a syringe barrel, as described in U.S. Pat. Nos. 3,469,581 and 3,542,024. However, the threads in such strip ejection sleeve have limited height and tapered rear surfaces to prevent substantial damage to the threads during strip ejection. Such threads have sufficient holding power to grip the needle hub's ears when assembled by a nurse or physician immediately prior to use. This is because there is no long term distorting pressure between the hub ears and threads.
Most disposable thermoplastic syringes are sold today with a needle preattached. The needle hub's ears and threads of the locking sleeve may be in high pressure abutment for several months during shipping, storage, etc. prior to use. This can cause some relief in the holding power of the threads on the needle hub ears.
In addition, strip ejected screw threads of limited holding power have problems with machine assembly of the needle to the syringe. If the needle is longitudinally jammed (without screwing) onto the tapered adapter to snap the hub's ears behind the threads, this causes some damage to the threads, reducing their already limited holding power. Machine assembly of the needle to the syringe with a screwing motion to firmly seat the needle on the tapered adapter sometimes causes the hub ears to snap out from behind the threads and form a scar or damage in the threads which can reduce the thread's holding power.
Threads in integral locking sleeves on syringes can be made with a greater height and more stability by unscrewing the syringe barrel and integral sleeve from its mold rather than using the longitudinal strip ejection process. However, such unscrewing process requires expensive mechanisms on the mold which substantially increase manufacturing cost.